One desirable type of photo detector is a two-color infrared radiation (IR) detector having simultaneous sensitivity in two spectral bands. The spectral bands may include short wavelength IR (SWIR), medium wavelength IR (MWIR), long wavelength IR (LWIR), and very long wavelength IR (VLWIR). An array of two color IR detectors may be employed in a number of imaging applications wherein it is required to simultaneously detect radiation within two spectral bands from a scene within a field of view of the array. By example, the array may be responsive to LWIR and MWIR, or to LWIR and SWIR. A two-color detector is disclosed in now commonly assigned U.S. Pat. No. 5,113,076, issued May 12, 1992, entitled “Two Terminal Multi-band Infrared Radiation Detector” to E. F. Schulte. Schulte discloses a radiation detector having two heterojunctions that function in a manner analogous to two back-to-back photodiodes. Each of the photodiodes is responsive to radiation within a different IR spectral band, such as LWIR and MWIR. Detection of a particular wavelength band is achieved by switching a bias supply. Disclosed configurations include an n-p-n configuration, a p-n-p configuration, and a p-n-p-n configuration. Reference is also made to now commonly assigned U.S. Pat. No. 5,149,956, issued Sep. 22, 1992, entitled “Two-Color Radiation Detector Array and Methods of Fabricating Same”, by P. R. Norton. This patent describes the formation of a substantially continuous common layer between semiconductor regions responsive to different wavelength bands (e.g., MWIR and LWIR). A contact is made to the common layer for coupling same to readout electronics. Reference is also made to now commonly assigned U.S. Pat. No. 5,380,669, issued Jan. 10, 1995, entitled “Method of Fabricating a Two-Color Radiation Detector Using LPE Crystal Growth”, by P. R. Norton. This patent describes the use of Liquid Phase Epitaxy (LPE) to grow an n-type LWIR layer, a p-type MWIR layer, and an n-type MWIR layer on a sacrificial substrate. A passivation layer is then formed over the n-type MWIR layer, an IR transparent substrate is bonded to the passivation layer, and the sacrificial substrate is then removed. The resulting structure is then further processed to form an array of two-color detectors.
Further in this regard reference can be had to now commonly assigned U.S. Pat. No. 5,457,331, issued Oct. 10, 1995, entitled “Dual Band Infrared Radiation Detector Optimized for Fabrication in Compositionally Graded HgCdTe”, by K. Kosai and G. R. Chapman.
Reference can also be made to commonly assigned U.S. Pat. No. 5,751,005, issued May 12, 1998, entitled “Low-Crosstalk Column Differencing Circuit Architecture for Integrated Two-color Focal Plane Arrays”, by Richard H. Wyles and William H. Frye. This patent describes an integrated two-color staring focal plane array having rows and columns of photo detector unit cells, each being capable of simultaneously integrating photocurrents resulting from the detection of two spectral bands.
Most existing two-color devices that are known to the inventor are based on two-junction structures that involve two p-n junctions per pixel. However, having two junctions requires two separate input circuits at the unit cell level. In small geometry and more complex designs, it is not always possible to place two complex circuits within a small footprint.
In commonly assigned U.S. Pat. No. 6,103,544, issued Aug. 15, 2000, entitled “Multiple Color Infrared Detector”, Peter D. Drieske and Sebastian R. Borrello describe a multiple color infrared detector formed from a photodiode, a photoconductor, and an insulating layer of material disposed between the photodiode and the photoconductor. The photodiode detects infrared radiation in the spectral band between about 3 microns and about 5 microns, and the photoconductor detects infrared radiation in the spectral band between about 8 microns and about 13 microns. While this multiple color device includes only a single p-n junction per pixel, a plurality of electrical contacts (112, 118, 122, 126) are used per pixel. In order to accommodate the multiple contacts, the optical fill factor is generally less than 75%.
Some existing small geometry pixel designs use a single-connection (single indium bump or more simply “single bump” design and rely on the readout integrated circuit (ROIC) unit cell to perform complex functions such as time division multiplexing (TDM). While this approach may satisfy many applications, it fails to meet other requirements such as gamma-suppression, or a footprint of less than 20 microns per unit cell.
Also of interest to this invention is now commonly assigned U.S. Pat. No. 5,731,621, issued Mar. 24, 1998, entitled “Three Band and Four Band Multispectral Structures Having Two Simultaneous Signal Outputs”, by Kenneth Kosai. This patent describes a solid-state array that has a plurality of radiation detector unit cells, where each unit cell includes a bias-selectable two-color detector or a single photo detector. Each unit cell is thereby capable of simultaneously outputting charge carriers resulting from the absorption of electromagnetic radiation within two spectral bands selected from one of four spectral bands or three spectral bands. In various embodiments a plurality of electrical contacts are used, such as a first, common contact, a second contact for outputting the bands 1 and 2 photocurrent, and a third contact for outputting the bands 3 and 4 photocurrent, or the band 3 photocurrent.
In the prior art, IR diodes are made with either N-on-P or P-on-N, where one of the layers is designed to absorb in the target energy range and the other layer is designed to suppress energy outside the target range. The function of the layers is altered by changing current direction, thereby changing the target energy bands absorbed and suppressed. What is needed in the art is a multi-color detector capable of simultaneously detecting energy in different bands without changing current direction, where the detector is amenable to manufacture at less than about 20 microns per unit cell.